1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a printer using the electrophotographic process or the electrostatic recording process.
2. Related Background Art
FIG. 8 schematically illustrates an example of a conventional image forming apparatus. The image forming apparatus is a transfer type electrophotographic apparatus, and is used as a copier, a printer, a facsimile machine, or the like.
The image forming apparatus has a photosensitive drum (drum-shaped electrophotographic photosensitive member) 101 as an image bearing member. The photosensitive drum 101 is driven to rotate counterclockwise as indicated by an arrow at a predetermined peripheral speed.
While the photosensitive drum 101 is in a rotating process, all of its surface is subject to pre-exposure by a pre-exposing device (an eraser lamp) 102 to erase any electric memory remaining in the previous image forming process. Then, a corona charger 103 as charging means uniformly charges the photosensitive drum 101 in a predetermined polarity at a predetermined potential. Then, an image exposure L is carried out by image exposing means (such as projection imaging exposure means of an original image or laser beam scanning exposure means) which is not shown in FIG. 8, and the charge of the uniformly charged surface of the photosensitive drum 101 is selectively eliminated (the potential is damped) so as to correspond to the pattern of the exposed image to form an electrostatic latent image on the surface of the photosensitive drum 101. The electrostatic latent image is developed by a developing device 104 as developing means and is visualized as a toner image.
On the other hand, by a sheet feeding mechanism which is not shown in FIG. 8, a transfer material (such as paper) P as a recording medium is supplied between the photosensitive drum 101 and a transfer corona charger 105 as transfer means at a predetermined timing. By charging the back surface of the transfer material P in the opposite polarity to that of the toner, the toner image on the photosensitive drum 101 is electrostatically transferred to the front surface of the transfer material P.
Then, the transfer material P is electrostatically separated from the surface of the photosensitive drum 101 by a separation corona charger 106, and is introduced into a fixing device which is not shown in FIG. 8 to fix the toner image. Then, the transfer material P is outputted as an object having an image formed thereon (a copy or a print). After the toner image is transferred from the photosensitive drum 101 to the transfer material P, toner remaining on the surface of the photosensitive drum 101 after the transfer is cleaned and removed by a cleaner 107 to use the photosensitive drum 101 in the subsequent image formation.
The above-described photosensitive member as the image bearing member, and the respective means and apparatus in the image forming processes such as the charging, exposure, development, transfer, fixing, and cleaning may have various kinds of constitutions and systems. For example, as the charging means, the above-described corona charger 103 is conventionally popularly used. The corona charger 103 is disposed so as to be opposed to and so as not to be in contact with the photosensitive drum 101 such that the surface of the photosensitive drum is exposed to corona discharged from the discharge wire of the corona charger 103 to charge the surface of the photosensitive drum in the predetermined polarity at the predetermined potential.
Recently, a contact type charger has been put to practical use which has advantages over the above-described non-contact type corona charger in that the amount of the generated ozone is less, the necessary electric power is lower, and the like. The contact type charger charges the photosensitive drum by making a charging member with voltage applied thereto in contact with the photosensitive drum. Among such contact type chargers, one having a magnetic brush as the contacting and charging member is preferably used in view of the stability of the contact with the photosensitive drum.
In the contact type charger using the magnetic brush, conductive magnetic particles are magnetically forced to be borne on a magnet or on a sleeve having a magnet encapsulated therein to form the magnetic brush. The magnetic brush is made to be in contact with the photosensitive drum while the magnetic brush is stopped or rotated, and, with voltage applied thereto, the photosensitive drum is charged.
As the contacting and charging member, other than the one described in the above, a conductive fur brush having a conductive fiber formed to be brush-like and a conductive rubber roller having conductive rubber formed in the shape of a roller are also preferably used.
In particular, when such a contacting and charging member is used in connection with a photosensitive drum having a surface layer with conductive particulates dispersed therein (a charge injection layer) on an ordinary organic photoconductor or a photosensitive drum using an amorphous silicon photoconductor, the charging of the surface of the photosensitive drum is carried out according to the injection charging method, and a potential which is substantially equivalent to a DC component of charging bias applied to the contacting and charging member can be charged on the surface of the photosensitive drum (U.S. Pat. No. 5,809,379).
Since the injection charging method does not use the discharge phenomenon as in the corona charging method, the photosensitive drum can be charged with no ozone being generated and with low electric power consumption, and thus, attention is being attracted to the injection charging method.
There is also a trend toward miniaturization of the image forming apparatus. However, miniaturization of the image forming apparatus as a whole is limited simply by making smaller the respective means and apparatus in the image forming processes such as the charging, exposure, development, transfer, fixing, and cleaning.
The toner remaining on the photosensitive drum 101 after the transfer is, as described in the above, cleaned by the cleaner 107 and is collected as waste toner. It is preferable that such waste toner is not generated in view of the environmental protection. Therefore, an image forming apparatus of the cleanerless system has appeared where the cleaner 107 is eliminated, the toner remaining on the photosensitive drum 101 after the transfer is removed by the developing device 104 according to the cleaning method simultaneous with developing, and the toner remaining after the transfer is collected in the developing device 104.
The cleaning simultaneous with developing is a method which collects a little amount of toner remaining on the photosensitive drum 101 after the transfer using fog removal bias (the potential difference Vback between the DC voltage applied to the developing device and the potential on the surface of the photosensitive drum) in developing processes for the next and subsequent image formations.
According to this method, the toner remaining after the transfer is collected in the developing device 104 to be used in development in the future, and thus, waste toner can be eliminated and trouble in the maintenance can be saved. Further, since there is no cleaner, the advantage from the viewpoint of space is also great, and the image forming apparatus can be miniaturized to a large extent.
When the method is combined with the magnetic brush charger, there is an advantage that, since the toner remaining after the transfer is collected by the magnetic brush charger and is reattached to the surface of the photosensitive drum substantially uniformly, by the developing device 104 is facilitated.
However, there are following problems particularly when a developing method is adopted using the cleanerless system in combination with the injection charging method by the magnetic brush charger and using a two-component developer having non-magnetic toner and magnetic carrier (magnetic particles) mixed therein as the developer.
To improve the charging efficiency of the magnetic brush charging, it is necessary to make lower the resistivity value of the magnetic particles for charging (PCF). On the other hand, if the resistivity value of the magnetic carrier used in the developer is low, charge injection is caused at a development nip portion, the fog removal potential Vback becomes smaller, and fog is caused. Therefore, from the viewpoint of prevention of the fog, it is necessary to make high the resistivity value of the magnetic carrier for developing.
In case of a cleanerless system, if the magnetic carrier in the developer unintentionally attaches to the photosensitive member, it is collected and accumulated in the magnetic brush charger. If the resistivity value of the magnetic carrier for developing is higher than that of the magnetic particles for charging, the resistivity value of the magnetic brush charger becomes higher, and its charging performance is lowered. For example, the photosensitive member can not be charged to a desired potential, or unevenness is caused in the charge.
As a method for compensating for this phenomenon, to make smaller the average particle diameter of the magnetic particles for charging is contemplated. However, in this case, the cohesiveness of the magnetic particles becomes greater, which leads to deteriorated conveyableness of the magnetic particles and deteriorated reattachableness to the photosensitive member of the toner collected in the magnetic brush charger. Therefore, the charging performance is lowered more remarkably as the image formation is repeated.